Digitally-synthesized loop filter circuit particularly useful for a phase locked loop

ABSTRACT

In a feedback system such as a PLL, the integrating function associated with a loop filter capacitor is instead implemented digitally and is easily implemented on the same integrated circuit die as the PLL. There is no need for either an external loop filter capacitor nor for a large loop filter capacitor to be integrated on the same integrated circuit die as the PLL. In a preferred embodiment, an analog phase detector is utilized whose phase error output signal is delta-sigma modulated to encode the magnitude of the phase error using a digital (i.e., discrete-time and discrete-value) signal. This digital phase error signal is “integrated” by a digital integration block including, for example, a digital accumulator, whose output is then converted to an analog signal, optionally combined with a loop feed-forward signal, and then conveyed as a control voltage to the voltage-controlled oscillator. The equivalent “size” of the integrating capacitor function provided by the digital integration block may be varied by increasing or decreasing the bit resolution of circuits within the digital block. Consequently, an increasingly larger equivalent capacitor may be implemented by adding additional digital stages, each of which requires a small incremental integrated circuit area. The power dissipation of the digital integration block is reduced by incorporating a decimation stage to reduce the required operating frequency of the remainder of the digital integration block.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Divisional application of U.S. Application No.09/902,541 filed on Jul. 10, 2001, now U.S. Pat. No. 6,630,868, entitled“Digitally-Synthesized Loop Filter Circuit Particularly Useful For APhase Locked Loop,” which application claims the benefit of U.S.Provisional Application No. 60/217,207, filed Jul. 10, 2000, and U.S.Provisional Application No. 60/217,208, filed Jul. 10, 2000. Each ofthese three applications is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to feedback systems, and particularly tothose circuits useful for implementing a phase locked loop, and moreparticularly to clock and data recovery circuits.

2. Description of Problem to be Solved and Related Art

Phase locked loops (PLLs) have been known and studied for quite sometime. Initially they were very expensive to implement, and found use inonly the most technically-demanding and/or cost-insensitiveapplications. However, as the cost of integrated circuit technology hasdecreased over the years, and as the performance capability of suchintegrated circuit technology has increased, today PLLs are extremelyinexpensive to implement and are found in wide use in many applications.

A generalized block diagram of a traditional PLL is shown in FIG. 1which is configured for a clock and data recovery application. The phaselocked loop 100 includes a phase/frequency detector 102 which receivesthe input data signal conveyed on node 112 and the output clock signalof the voltage controlled oscillator (VCO) 110 conveyed on node 124. Thephase/frequency detector 102 generates on its output node 116 an errorsignal which is a function of the phase difference between the inputdata signal and the VCO clock, and may also include additional circuitryto generate on an output node 114 the reconstructed data, as shown.

A gain block 104, an integrator block 106, and a summer block 108together form a filter block which low-pass filters the output of thephase/frequency detector 102 to generate a control signal on node 122which is provided to the voltage controlled oscillator 110 in order toinfluence the frequency (and hence the phase) of the VCO output signal.The integrator block 106 is often implemented using a charge pump and aloop filter capacitor, as is well known in the art. Such loop filtercapacitors are usually required to be very large for the PLL to exhibitacceptable peaking behavior in its frequency response.

In order to appreciate this issue, a brief description of the frequencyresponse of this traditional PLL is warranted. The closed loop transferfunction, G(s), of this traditional PLL 100 is set forth in Equation 1:$\begin{matrix}{{G(s)} = \frac{\frac{K^{\prime}}{\omega_{z}}\left( {s + \omega_{z}} \right)}{S^{2} + {\frac{K^{\prime}}{\omega_{z}}s} + K^{\prime}}} & \left( {{Eq}.\quad 1} \right)\end{matrix}$

where K′ and ω_(z) are determined by the settings of various PLLparameters. In the traditional PLL 100, the value of ω_(z) is given byEquation 2. $\begin{matrix}{\omega_{z} = \frac{I}{CK}} & \left( {{Eq}.\quad 2} \right)\end{matrix}$

where I corresponds to the magnitude of the current of the charge pump,C corresponds to the magnitude of the loop filter capacitor, and Kcorresponds to the gain of the gain block 104. A graph of the frequencyresponse of this closed loop transfer function G(s) is shown in FIG. 2by curve 130. As shown in this graph, the magnitude of the transferfunction is fairly constant at low frequency, and increases slightly forfrequencies between ω_(z) and ω_(BW) (which corresponds to the bandwidthof the closed loop transfer function). As frequency increases aboveω_(BW), the magnitude of the transfer function falls off rapidly. This“peaking” region in the transfer function is labeled as 132.

The magnitude of this peaking is very critical for many applications.For example, the SONET specification limits the acceptable peaking to0.1 dB. If allowed to exceed this limit, frequency components of inputdata jitter which fall within this peaking region are actually amplifiedby the PLL. If several such PLLs are coupled sequentially, the jittermay be amplified to a degree which severely compromises the ability tomeet jitter tolerances, or even to correctly recover data.

If we define: $\begin{matrix}{\frac{\omega_{BW}}{\omega_{z}} = \frac{\gamma}{\gamma - 1}} & \left( {{Eq}.\quad 3} \right)\end{matrix}$

From the SONET specification of 0.1 dB, we arrive at a value of gamma of1.01. Consequently, $\begin{matrix}{\frac{\omega_{BW}}{\omega_{z}} = 101} & \left( {{Eq}.\quad 4} \right)\end{matrix}$

and $\begin{matrix}{\frac{I}{CK} = \frac{\omega_{BW}}{101}} & \left( {{Eq}.\quad 5} \right)\end{matrix}$

For the OC48 data rate of the SONET specification, the loop bandwidthmust meet the following relationship:

ω_(BW)≦2π2 MHz  (Eq. 6)

The magnitude of the gain factor K is set by the loop bandwidth and theVCO gain, K_(V), and is typically much less than unity, such as, forexample: $\begin{matrix}{\frac{4\pi}{50} \cong 0.25} & \left( {{Eq}.\quad 7} \right)\end{matrix}$

To achieve a reasonably fast charge pump in, for example, 0.25 μsemiconductor technology, the value of I may be advantageously set to100 μA. Calculating for the required magnitude of the loop filtercapacitor, we arrive at: $\begin{matrix}\begin{matrix}{C \geq {101\frac{I}{K}\left( \frac{1}{\omega_{BW}} \right)}} \\{{C \geq {100\frac{100\quad {µA}}{0.25}\left( \frac{1}{2\quad {\pi 2}\quad {MHz}} \right)}} = {3.2\quad {nF}}}\end{matrix} & \left( {{Eq}.\quad 8} \right)\end{matrix}$

This amount of capacitance (3.2 nF) is difficult to integrate onto anintegrated circuit without requiring large amounts of die area for thecapacitor. For lower data rates, an even greater amount of capacitanceis required (e.g., 16 times as much for OC3). For this reason, the loopfilter capacitor is usually provided externally. But such an externalcapacitor adds an additional complexity to board layout, and introducesnoise susceptibility on the extremely critical loop filter node withinthe PLL.

There have been other attempts to reduce the size of the required loopfilter capacitor. One such method is described by Bulzachelli in U.S.Pat. No. 5,036,298 in which the input data signal is routed through avariable delay block, whose output is then routed to the phase detector.This results in a zero placed in the loop feedback path that does notappear in the closed loop transfer function, and hence there is nopeaking in the closed loop transfer function. The large filter capacitorotherwise required at least partially to achieve acceptably low peakingis not required to be as large. While this is an elegant engineeringsolution, there are nonetheless difficulties which must be dealt with toimplement such a solution requiring a variable delay block. First, itmay be difficult to implement a variable delay block having an adequatedelay range, especially in multi-rate applications. Additionally, thevariable delay block must accurately delay the data signal in spite ofthe random nature of data transitions in the data signal, where the timebetween transitions is not necessarily constant. Moreover, the variabledelay block represents yet another block of circuitry that must operateat the fill data rate, and consequently its power dissipation may not beinsignificant, especially when a low power clock and data recoveryimplementation is desired.

In spite of these previous efforts, and notwithstanding the long historyof engineering efforts refining the design of phase locked loops, mostPLLs still require either a large external capacitor or requiresignificant additional integrated circuit die area to implement the loopfilter capacitor monolithicly. Therefore, additional improvements whichcan reduce the size of the loop filter capacitor are still greatlydesired.

SUMMARY OF THE INVENTION

In a feedback system, such as a PLL, the integrating function associatedwith a loop filter capacitor may be implemented digitally rather thanusing a traditional integrating capacitor. The area required for such adigital integrating block is easily implemented on the same integratedcircuit die as the PLL. There is no need for either an external loopfilter capacitor nor for a large loop filter capacitor integrated on thesame integrated circuit die as the PLL. Consequently, printed wiringboard layout issues are simplified, and at least one dedicated packagepin may be eliminated. Other kinds of feedback systems can also benefitby implementing a loop filter capacitor function or other long timeconstant requirement by digitally synthesizing the integratingcapacitor.

In certain embodiments of the invention an analog phase detector may beutilized, whose phase error output signal is converted to a digitalsignal by an analog-to-digital (A/D) converter. In other embodiments adigital phase detector may be utilized whose phase error output signalis already a digital signal. The digital phase error signal may bedigitally “integrated” by a digital integration block including, forexample, a digital accumulator block, whose output is then convertedback to an analog signal, filtered, optionally combined with a loopfeed-forward path signal, and then conveyed as a control voltage to thevoltage-controlled oscillator. The equivalent “size” of the integratingcapacitor function provided by such an arrangement may be varied byincreasing or decreasing the number of bits within the digitalaccumulator block. For example, the number of bits may be changed toadjust a loop filter for different incoming data rates or inputfrequencies. Consequently, an increasingly larger equivalent capacitormay be implemented by adding additional digital stages, each of whichconsumes low power and requires a small incremental integrated circuitarea.

The required resolution of the digital accumulator output may be lessthan the number of bits in the accumulator, which allows the lower orderbits to be decimated. The high order portion of such a digitalaccumulator may then be operated at a far lower clock rate than lowerorder portions, thus reducing power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1, labeled as prior art, is a block diagram of a traditional phaselocked loop arranged to perform a clock and data recovery function.

FIG. 2, labeled as prior art, is a graph depicting the closed looptransfer function of the PLL shown in FIG. 1.

FIG. 3 is a block diagram of an embodiment of a phase locked loopcircuit incorporating a digitally-synthesized loop filter capacitorcircuit in accordance with the present invention.

FIG. 4 is a block diagram of a model for the overall loop filter for thephase locked loop circuit shown in FIG. 3.

FIG. 5 is a block diagram of another embodiment of a phase locked loopcircuit incorporating a digitally-synthesized loop filter capacitorcircuit in accordance with the present invention.

FIG. 6 is a conceptual block diagram of a digital phase detectorincorporating a delta-sigma modulator.

FIG. 7A is a schematic diagram of an embodiment of a linear phasedetector useful within the digital phase detector shown in FIG. 6.

FIG. 7B is a schematic diagram of another embodiment of a linear phasedetector useful within the digital phase detector shown in FIG. 6.

FIG. 7C is a schematic diagram of a logic circuit useful for the phasedetector shown in FIG. 7B.

FIG. 7D is a schematic diagram of an embodiment of a digital phasedetector in accordance with the present invention.

FIG. 8 is a block diagram of the phase detector conceptually depicted inFIG. 6.

FIG. 9 is a block diagram of another representation of a linearizedmodel of the phase detector depicted in FIG. 6.

FIG. 10 is a block diagram of a portion of the phase locked loop circuitshown in FIG. 5.

FIG. 11 is a block diagram of a portion of the phase locked loop circuitshown in FIG. 5, which illustrates a digital integrating pathincorporating a decimation block.

FIG. 12 is a block diagram of a useful decimation block for the circuitportion shown in FIG. 11.

FIG. 13A is a schematic diagram of one embodiment of the decimationblock shown in FIG. 12.

FIG. 13B is a schematic diagram of an embodiment of a digitalaccumulator block shown in FIG. 11.

FIG. 13C is a block diagram of an exemplary portion of a PLL circuitincluding a digital phase detector and a digitally-synthesized loopfilter capacitor in accordance with the present invention.

FIG. 14 is a block diagram of a clock and data recovery circuitincorporating the present invention, and for which the present inventionis particularly advantageous.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

An exemplary PLL incorporating a digital integrating block in accordancewith the present invention, and which is configured for a clock and datarecovery application, is shown in FIG. 3. The phase locked loop 140includes an analog phase detector 142 which receives the input datasignal conveyed on node 164 and the output clock signal of the voltagecontrolled oscillator (VCO) 162 conveyed on node 184. The analog phasedetector 142 generates on its output node 168 an error signal whichvaries according to the phase difference between the input data signaland the VCO output clock signal, and may also include additionalcircuitry to generate on an output node 166 the reconstructed outputdata, as shown. Alternatively, such data sampling circuitry may beimplemented external to the analog phase detector 142.

The loop filter for this exemplary PLL 140 includes a feed forward pathformed by a gain block 144 and a filter block 146, and further includesa digital integrating block 152. The output of the feed forward path,which is conveyed on node 172, and the output of the digital integratingblock 152, which is conveyed on node 180, are combined by summer block150 to generate a control signal on node 182 for the voltage controlledoscillator 162 in a manner similar to the above described PLL shown inFIG. 1. Alternatively, the VCO 162 may include two different controlinputs, connected respectively to nodes 172 and 180, thus eliminatingthe need for a separate summer block.

The digital integrating block 152 includes an A/D converter 154, adigital accumulator 156, a D/A converter 158, and a filter block 160.The A/D converter 154 receives the analog phase error output signal fromthe analog phase detector 142, which is conveyed on node 168, andgenerates on its output node 174 a digital representation of the analogphase error voltage. The digital accumulator 156, which includes amultiple-bit register to represent the cumulative (i.e., integrated)value of the phase error, takes each digital phase error representationfrom the A/D converter 154 and increases or decreases the cumulativevalue accordingly. For example, if the digital phase errorrepresentation corresponds to a “positive” voltage, the digitalaccumulator 156 will increase the cumulative value stored in its outputregister. Conversely, if the digital phase error representationcorresponds to a “negative” voltage, the digital accumulator 156 willdecrease its cumulative value stored in its output register. In thecontext used here, “positive” and “negative” values merely areunderstood to mean values relative to a neutral (no phase error) valueof the analog phase error signal. The actual voltages may or may not bepositive or negative with respect to a ground reference voltage.Frequently, the analog phase error signal conveyed on node 168 is adifferential signal (conveyed, in that case, on a pair of output nodes168), and “positive” and “negative” merely refers to the polarity ofsuch a signal. As will be described below, a variety of suitable A/Dconverter structures may be used.

The multiple-bit output register of the digital accumulator 156 holds adigital representation of an integrated value of the phase error,analogous to the function usually performed by a charge pump and a largeloop filter capacitor. This digital value is communicated on an N-bitwide output bus 176 to the D/A converter 158 which converts the digitalrepresentation back into an analog signal conveyed on its output node178, whereupon the filter block 160 provides a smoothing function to thereconstructed analog signal. The output of the filter block 160 is thenconveyed on node 180 to the summer block 150 (or alternatively, directlyto the VCO 162).

The digital accumulator 156 may be implemented using any of a variety ofstructures. For example, an adder may be used if the digitalrepresentation conveyed on node 174 includes an appropriate polarityindication of the digital value represented. Alternatively, variouscounter structures (e.g., an up/down counter) may also be utilized, asfurther described below.

Referring now to FIG. 4, a model of the overall loop filter describedabove is represented. The input node to the loop filter is node 168(which receives the analog phase error signal), and the output of theloop filter is node 182, upon which a control signal for the VCO 162 isconveyed. The forward gain block 144 and the summer block 150 also servewell as their respective models. The A/D converter 154, the digitalaccumulator 156, and the D/A converter 158 collectively are modeled bythe pair of elements 192 and 194. (The filter block 160 may be ignoredin the model since it can be designed to have negligible influence onthe PLL dynamics.) Model block 192 represents an attenuation resultingfrom the digital accumulator 156 which increases as the bit-width of thedigital accumulator 156 increases, while model block 194 represents anaccumulator or digital integrator structure which adds its previousvalue to the present input value to arrive at a new present value.

The loop filter transfer function of this loop filter may be written as:$\begin{matrix}{{H(z)} = {K_{f} + {\frac{1}{K_{1}}\left( \frac{1}{1 - z^{- 1}} \right)}}} & \left( {{Eq}.\quad 9} \right)\end{matrix}$

This expression may be simplified by observing that, at low frequencies(i.e., <<1/T): $\begin{matrix}\left. \frac{1}{1 - z^{- 1}}\Rightarrow{\frac{1}{1 - ^{{- {j2}}\quad \pi \quad {fT}}} \approx \frac{1}{j\quad 2\quad \pi \quad {fT}}} \right. & \left( {{Eq}.\quad 10} \right)\end{matrix}$

where T represents the sampling period of the analog-to-digitalconversion (and implicitly the period of the digital accumulator aswell). Consequently, the transfer function may be re-written as:$\begin{matrix}\begin{matrix}{{H(z)} = {K_{f} + {\frac{1}{K_{1}}\left( \frac{1}{1 - ^{{- {j2}}\quad \pi \quad {fT}}} \right)}}} \\{{H(z)} \approx {K_{f} + {\frac{1}{K_{1}}\left( \frac{1}{j\quad 2\quad \pi \quad {fT}} \right)}}} \\{{H(s)} = \frac{1 + {{s \cdot K_{f}}K_{1}T}}{{s \cdot K_{1}}T}}\end{matrix} & \left( {{Eq}.\quad 11} \right)\end{matrix}$

A zero may be achieved in this transfer function at a frequency of 10kHz or below (as per the OC-48 SONET specification) when the followingrelation is true: $\begin{matrix}{{K_{f}K_{l}T} \geq \frac{1}{2\quad {\pi \left( {10\quad {kHz}} \right)}}} & \left( {{Eq}.\quad 12} \right)\end{matrix}$

This relationship confirms that a zero may be achieved at suitably lowfrequencies without the need for a large capacitor by simply setting thevalue of K_(f)K₁T to an appropriate value. Moreover, since a large loopfilter capacitor is not required, there is no need for either anexternal loop filter capacitor nor for a large loop filter capacitorintegrated on the same integrated circuit die as the PLL. The variousother filter-related capacitors are much smaller in size and may beeasily integrated on-chip. Consequently, printed wiring board layoutissues are simplified, and at least one dedicated package pin iseliminated. Moreover, since a traditional loop filter capacitor iscoupled to a node that is highly sensitive to potential noise (any noisequickly propagates to the VCO control node), the potential noise sourceassociated with coupling the loop filter capacitor node from off-chip iseliminated. As a result, a PLL incorporating a digital integrating blockas described above promises to be less sensitive to noise thantraditional designs incorporating a loop filter capacitor.

Referring now to FIG. 5, another embodiment of a PLL is shown which alsoincorporates a digital integrating block. In this embodiment, a phasedetector having a digital output signal is used, which eliminates therequirement for a separate analog-to-digital converter otherwiserequired in the loop integrating path. The phase locked loop 200includes a digital phase detector 202 which receives the input datasignal conveyed on node 220 and the output clock signal of the voltagecontrolled oscillator 218 conveyed on node 236. The digital phasedetector 202 generates on its output node 222 an error signal whichdigitally encodes the phase difference between the input data signal andthe VCO output clock signal. The digital phase detector 202 may alsoinclude additional circuitry to generate on an output node 238 thereconstructed output data, as shown.

The loop filter for this exemplary PLL 200 includes a feed forward pathformed by a gain block 204 and a filter block 206, and further includesa digital integrating block 210. The output signal of the feed forwardpath, which is conveyed on node 226, and the output signal of thedigital integrating block 210, which is conveyed on node 234, arecombined by summer block 208 to generate a control signal on node 228for the voltage controlled oscillator 218.

The digital integrating block 210 includes a digital accumulator 212, aD/A converter 214, and a filter block 216. The digital accumulator 212,which conceptually includes a summer 213 and a multiple-bit register 215to represent the cumulative (i.e., “integrated”) value of the phaseerror, receives each digital phase error representation from the digitalphase detector 202, which is conveyed on node 222, and increases ordecreases the cumulative value accordingly, as described above. Forexample, if the digital phase error representation corresponds to a“leading” phase relationship, the digital accumulator 212 will increase(or alternately, decrease) the cumulative value stored in its outputregister. Conversely, if the digital phase error representationcorresponds to a “lagging” phase relationship, the digital accumulator212 will decrease (or alternately, increase) its cumulative value storedin its output register. While this and other block diagrams aredescribed using the terminology of a single node connecting the blocks,it should be appreciated that, when required by the context in thevarious embodiments, such a “node” may actually represent a pair ofnodes for conveying a differential signal, or may represent multipleseparate wires (e.g., a bus) for carrying several related signals.

As described above, the multiple-bit output register of the digitalaccumulator 212 holds a digital representation of an integrated value ofthe phase error. This digital value is preferably communicated on anN-bit wide output bus 230 to the D/A converter 214 which converts thedigital representation back into an analog signal conveyed on its outputnode 232, whereupon the filter block 216 provides a smoothing functionto the reconstructed analog signal. The output of the filter block 216is then conveyed on node 234 to the summer block 208 (or alternatively,directly to the VCO 218).

As stated above, because a phase detector having a digital output signalis used, the requirement for a separate analog-to-digital converter,which exists only in the digital integrating block, is eliminated. Thisallows a low offset to be achieved in the phase error through the feedforward path because both the feed forward path and the integrating pathreceive the same digital signal. As used herein, a digital phasedetector is one having an output signal which is quantized in time andquantized in value, even if such digital output signal linearly encodesthe phase error.

It should be understood that a classical implementation of a digitalphase detector, such as a “bang-bang” phase detector, has severaldisadvantages, at least for certain applications. Such a phase detectorarrangement requires registers within the phase detector to operate witha very narrow metastability window, otherwise the input jitter and phaseoffset are all the more accentuated, due to the phase detector'sinability to encode small amounts of phase error. Moreover, such abang-bang detector, whose output polarity is steered by the polarity ofthe phase error, but whose output magnitude and duration (per clockcycle) is fixed irrespective of the actual magnitude of the phase error,gives rise to non-linear PLL dynamics. Nontheless, such a phase detectormay be utilized in certain embodiments. An exemplary bang-bang phasedetector is described in “Clock Recovery from Random Binary Signals,” J.D. H. Alexander, Electronics Letters, Vol. 11, pp. 541-542, Oct. 1975,which is hereby incorporated by reference.

A preferable digital phase detector is depicted conceptually in FIG. 6.This exemplary digital phase detector 202 includes, for this embodiment,a linear phase detector 240 followed by a first-order delta-sigmamodulator 242. The linear phase detector 240 compares the phase of theinput data signal conveyed on node 220 to the phase of the data clocksignal (which may be the VCO clock or a divided-down version thereofwhen used in a multi-rate device) conveyed on node 236, and generates anoutput signal that varies substantially linearly with phase differencebetween its input signals, at least over a certain range of phasedifference (e.g., approximately −π to +π). Preferably, a linear phasedetector output signal has either or both an amplitude or a pulse widththat varies substantially linearly with phase difference. Even morepreferably, the output signal is a pulse width modulated error signalwaveform. In this example, the error signal is a current waveformflowing into or out of node 250, although a voltage signal may also beemployed in other circuits.

The delta-sigma modulator 242 then converts the pulse width modulatederror signal into a discrete-time and discrete-amplitude digital outputsignal, in this example generating a one-bit digital output on itsoutput node 222. The delta-sigma modulator 242 includes a modest-sized(e.g., having a typical value of 2-3 pF) integrating capacitor 248connected to node 250, and further includes a digital comparator block244 which samples the voltage on its input node 250 when clocked by adelta-sigma clock received on clock node 254. Such a comparator block244 preferably includes a gain stage followed by a register. The digitaloutput generated on the output node 222 is fed back as a negativecurrent by feedback block 246 into node 250 to provide the requisitefeedback into the integrating capacitor 248 of the delta-sigmamodulator. Operation of such first-order delta-sigma modulators arewell-known to one skilled in the art. Suitable clock rates for thedelta-sigma clock are described in greater detail herebelow, butpreferably are set high enough to reduce quantization noise influence onthe feedforward path, yet low enough so that the latching circuit withinthe comparator resolves when strobed by the comparator clock (i.e., the“delta-sigma clock”).

An advantageous linear phase detector 240 is illustrated in FIG. 7A.Similar circuits are generally well known in the art. In this circuit, aregister 260 samples the input data signal conveyed on node 220 whenclocked by the data clock signal conveyed on node 236. The first XORgate 266 generates on its output node 276 a variable-width pulse ofduration generally equal to the time by which an input data signaltransition leads the corresponding transition on node 272, which iscontrolled, of course, by the data clock. The delay block 262 isincluded to compensate for the clock-to-Q delay of the register 260.When the data clock is correctly aligned to the input data signal (i.e.,data clock transitions at precisely the mid-point of the databit-intervals), the register 260 generates on its output node 272 asignal that replicates the input data signal, but delayed by one-halfperiod of the data clock, and the pulse on node 276 is of a durationexactly equal to one-half period of the data clock.

The latch 264 generates on its output node 274 a signal which replicatesits input signal on node 272, but delayed by one-half period of the dataclock. As a result, the second XOR gate 268 generates on its output node278 a pulse with a duration that is always equal to one-half the periodof the data clock. The fixed-duration pulse signal conveyed on node 278is subtracted from the variable-width pulse signal conveyed on node 276by summing block 270. When the data clock is correctly aligned, bothpulse signals have equal durations, and the summing block 270 generatesa zero-valued net error current. If the input data transition arrivestoo early, the pulse signal on node 276 is longer than the pulse signalon node 278, and a net error current is generated by the summing block270. Obviously, the remainder of the PLL is arranged to respond to suchpolarity of error current in a direction to advance the phase of thedata clock. Preferably, an additional latch (not shown) is includedbetween the register 260 and the latch 264 to insulate the earliersignal entering the XOR gate 268 from variations in the timing of node272 resulting from varying input data timing (i.e., variations inclock-to-Q timing of register 260 as a function of its input data setuptime). In such a configuration, the two inputs of the second XOR gate268 are still preferably taken from the input and output nodes of thelatch 264.

Such an improved linear phase detector 490 is illustrated in FIG. 7B. Alatch 275 is shown connected between the register 260 and the latch 264,and the inputs to the second XOR gate 268 are taken from the input andoutput nodes of latch 264, being nodes 273 and 274. As a result, thesecond XOR gate 268 still generates on its output node 278 a pulse witha duration that is always equal to one-half the period of the dataclock, but is delayed by an additional half-clock period. Described inanother fashion, the latch 275 is included between the first pulsegeneration circuit (comprising register 260 and XOR gate 266) and thesecond pulse generation circuit (comprising register 264 and XOR gate268) to improve the accuracy of the one-half data clock period pulsegenerated by the second pulse generation circuit, and thus to improvethe gain uniformity of the phase detector.

Another latch 277 is shown having an input coupled to node 274, forconveying the recovered re-timed data on an output node 279. Such arecovered output data could be taken from any of several latch outputnodes (e.g., nodes 272, 273, or 274) but by including an additionallatch 277, the capacitive loading on each of these other latch outputnodes may be made lower in magnitude and more accurately matching theother latch output nodes. As a consequence, better matching within thetwo pulse generation circuits within the phase detector 490 results inmore accurate pulses and a lower static phase error.

While shown in FIG. 7A and FIG. 7B using single-ended logic blocks andsignals, in practice such circuits are preferably implemented usingfully differential circuitry. This provides enhanced noise immunity,better speed, and more consistent delays which are independent of datastate. Moreover, many of the circuit blocks, such as the summing block270, are more easily implemented and achieve better matching of currentswhen implemented differentially, thereby resulting in lower offsets. Inparticular, the summing block 270 may be advantageously implemented by a“wire-or” connection directly between the outputs of logic gates 266 and268 to combine the two output signals when such signals are currentsignals. In such a case, the summing block 270 may be a common loadcircuit for the common output nodes(s). In other embodiments, thesumming block 270 may be implemented as a more distinct circuit.

Referring now to FIG. 7C, a preferred embodiment of a differential XORgate (e.g., gate 266, 268) is depicted. True and complement inputs for afirst input A and a second input B are conveyed to the respective gateterminals of various N-channel metal-oxide-semiconductor (NMOS)transistors. A first level differential transistor pair includestransistor 502 and 504 which receive the B and complement-B signalsrespectively. A second level includes a first differential transistorpair 506 and 508 which receives the A and complement-A signals,respectively, and a second differential transistor pair 510 and 512which receive the complement-A and A signals, respectively. Adifferential output current is conveyed on a pair of differential outputnodes 514 and 516 in accordance with the XOR function of the two inputsA and B.

Referring now to FIG. 7D, a schematic diagram of a preferred embodimentof a digital phase detector 520 is shown, which includes a linear phasedetector (such as linear phase detector 490) and a delta-sigma modulatorto produce a digital phase detector output signal. The differentialcurrent outputs from XOR gates 266 and 268 are combined by directlyconnecting the outputs together, with the true polarity output from gate266 and the complement output from gate 268 being coupled to node 530,and the with the true polarity output from gate 268 and the complementoutput from gate 266 being coupled to node 532. Consequently, only theinternal nodes of the XOR gates need operate with a pulse width on theorder of half a clock period. A load circuit 534 provides a cascodeconstant current source load structure for each of nodes 530 and 532. Inparticular, transistors 536 and 537 are biased by a voltage conveyed onnode 542 which is approximately the common-mode voltage of nodes 530 and532. Cascode transistors 538 and 539 are included in series withtransistors 536 and 537 to provide the load circuit 534 with a higheroutput impedance (i.e., more uniform current magnitude as a function ofoutput voltage). The bias voltage on node 542 is generated bytransistors 540 and 541 functioning as a resistive divider, whichtransistors are preferably long channel, narrow width PMOS transistorsto preserve the high output impedance on nodes 530 and 532.

The delta-sigma modulator includes a pair of integration capacitors 544and 546 connected respectively to the phase detector output nodes 530and 532, a comparator circuit 520 having a differential input paircoupled to the phase detector output nodes 530 and 532, and a feedbackcircuit 526 having differential inputs coupled to the differentialoutputs of the comparator circuit 520 and having differential outputsfeeding back and connected respectively to the phase detector outputs530 and 532. The feedback circuit 526 includes a differential transistorpair (with an associated current source). The delta-sigma modulator alsoincludes a second differential transistor pair (with an associatedcurrent source) to provide a differential output signal on nodes 554 and556.

Several other transistors are provided to support a calibrationcapability of a PLL incorporating such a digital phase detector.Transistors 548, 549, 550, and 551 are provided to force a full-scalehigh signal or a full-scale low signal onto phase detector output nodes530 and 532. When a POS_RAIL signal is asserted, transistor 549 drivesnode 530 toward ground (i.e., the “negative rail”) and transistor 550drives node 532 toward V_(DD) (i.e., the “positive rail”). Similarly,when a NEG_RAIL signal is asserted, transistor 551 drives node 532toward ground and transistor 548 drives node 530 toward V_(DD). In thesecases, the delta-sigma modulator will generate an output signalcorresponding to full scale error signals from the linear phase detectorirrespective of the actual phase error between the input signals, andthe difference in frequency of the VCO resulting in response to thesetwo different signals may be observed to compute the gain of the PLL.

A third calibration signal MID_RAIL is also shown which forces thedelta-sigma modulator to generate an alternating string of 1, −1, 1, −1,etc. on its output nodes 554 and 556. The comparator circuit 520includes a multiplexer 522 (which also provides a gain of preferablyabout 3) which couples the outputs of register 524 (i.e., the outputs ofthe comparator 520) back to the inputs of the register 524 with areversed polarity, so that the register 524 “oscillates” with each clockedge of a DS_CLK conveyed on the delta-sigma clock node 558. Atransistor 552 is turned on during this mid-rail mode to keep the phasedetector output node 530, 532 moderately well-behaved even though thefeedback circuit 526 is conveying alternating current signals into thenodes.

Referring now to FIG. 8, a block diagram of the phase detector 202 isillustrated. The summing block 280 and gain block 282 correspond to thelinear phase detector 240, while the summing block 284, integrator block286, comparator block 288, and feedback block 290 correspond to thedelta-sigma modulator 242. This model may be represented in the formshown in FIG. 9, in which the delta-sigma modulator 242 is modeledinstead by a single block 294 and a summer 296 to include the effects ofshaped quantization noise arising from the delta-sigma modulation. Thesize of the integration capacitor and the magnitude of feedback current(and the linear phase detector currents) are preferably chosen togenerate a voltage “ripple” on the delta-sigma integration node (i.e.,the input node) which keeps the comparator circuit generally operatingoutside its metastability region, yet is not so large a ripple to causenon-linearities on the currents being summed.

The benefits of using a digital phase detector, such as the exemplaryone described above in relation to FIG. 7D, are several-fold. Of note,the PLL dynamics are linear because the digital phase detector encodeson its output a signal whose value represents a pulse-width modulatedphase error signal which linearly varies as a function of the phasedifference of the input data signal compared to the data clock. Thisallows a more straightforward design of the PLL to meet the desiredjitter transfer and jitter tolerance specifications. Moreover, noregisters within the phase detector are operated, when the data clock isproperly aligned, in their metastable region (as occurs with atraditional bang-bang phase detector). This allows registers with awider metastable region to be used without significant jitter penalty.

Having described the overall organization of the exemplary PLL thus far,various issues affecting implementation of the digital integrating block(e.g., block 210) are now described. Referring now to FIG. 10, a portion300 of PLL 200 is depicted which includes the digital phase detector 202and the digital integration block 210 from earlier FIG. 5. As can beappreciated, the delta-sigma modulator 242 within the digital phasedetector 202 is clocked by a delta-sigma clock, and consequently theoutput signal on node 222 is synchronized to the delta-sigma clock. Inorder to minimize the impact of quantization noise within thedelta-sigma modulator (which can be in the GHz range), this delta-sigmaclock should be set to as high a clock rate as possible. For aparticular implementation using 0.25 μ semiconductor technology with amaximum data clock of approximately 2.5 GHz, this delta-sigma clock isadvantageously operated at half the data clock rate, or 1.25 GHz. Inprinciple, the digital integration block 210 must also be clocked at thesame delta-sigma clock rate since it receives the output signal conveyedon node 222. Unfortunately, operating such circuits as a digitalaccumulator having, for example, a dozen or more bits of resolution, ata clock rate of, for example, 1.25 GHz leads to extremely high powerdissipation in the digital integrator.

Recall the model of the overall loop filter incorporating a digitalintegrating block, as shown in FIG. 4. To achieve a zero in the loopfilter transfer function of less than 10 kHz (for OC48), the followingrelation was found:${K_{f}K_{l}T} \geq \frac{1}{2\quad {\pi \left( {10\quad {kHz}} \right)}}$

For a 1.25 GHz delta-sigma clock rate, the value of K_(f)K₁ is found tobe:${{K_{f}K_{l}} \geq \frac{1}{2\quad {\pi \left( \frac{1}{1.25 \times 10^{9}} \right)}\left( {10 \times 10^{3}} \right)}} = {19\text{,}894}$

This represents a number greater than 2¹⁴. Without any loss ofgenerality, we can assume that K_(f)=1. Consequently, the value of K₁must be greater than 2¹⁴. The digital accumulator therefore should be atleast 15 bits wide since an extra bit is required due to the bilateralrequirement of the accumulator range. For lower data rates, an evengreater accumulator width is required (e.g., 19 bits for OC3 datarates). Since, for this example, the digital phase detector output is aone-bit signal, the resolution of the digital accumulator 212 istherefore greater than 2⁻¹⁴ UI (i.e., unit interval). This resolution ismuch greater than that required to achieve the SONET jitterspecifications. For example, a resolution of only 2⁻¹⁰ UI would beadequate.

Referring now to FIG. 11, an improved arrangement is shown which “throwsaway” the unnecessary resolution to substantially lower the total powerdissipation of the digital integrating block. In this arrangement, adecimation stage 312 is preferably included in the path between thedigital phase detector 202 and a digital accumulator 212, which allowsthe digital accumulator 212 to be clocked by an accumulator clock(conveyed on node 316) which has a much lower clock rate than thedelta-sigma clock. This results in a significant savings in powerdissipation of the digital accumulator 212, and even allowing for thedissipation of the decimation stage 312, a savings in total powerdissipation of the digital integrating block. Moreover, a lower speeddigital accumulator design is much simpler to implement and moreamenable to use of automatic synthesis tools for both its design andlayout. It can be designed with virtually any number of bits ofresolution without significant increase in power and without muchincrease in layout area. In other embodiments, a separate decimationstage 312 is not utilized, and the digital accumulator 212 is connecteddirectly to the output of the digital phase detector 202. In such aconfiguration, a portion of the digital accumulator may itself functionas a decimation circuit, as described below.

To arrive at suitable implementations of the decimation stage 312,recall that the output signal from the digital phase detector 202 is anoutput sequence of ones and zeros generated by the delta-sigma modulatorwithin the digital phase detector. Any transition of the output signalfrom a one to a zero or from a zero to a one occurs just after theactive transition of the delta-sigma clock. Moreover, if the outputsignal is sampled during each “bit interval” or period of thedelta-sigma clock, the number of ones compared to the number of zeros(summed over a number of clock periods) encodes the analog phase errorvoltage. For example, a higher voltage results in a larger number ofones (compared to zeros) than does a lower voltage (i.e., a largerones-density in the signal).

One such suitable decimation stage which takes advantage of thisdelta-sigma output signal characteristic is shown in FIG. 12 (otherdecimation circuits are described below). The decimation stage 312includes a 1-to-transition converter 320, a divide-by-2^(N) block 322,and a transition-to-1 converter 324. The 1-to-transition converter 320samples the delta-sigma output signal of the phase detector preferablyduring each period of the delta-sigma clock (which corresponds to thedata interval of the delta-sigma output signal) although sampling atother predetermined intervals is also contemplated. When the phasedetector output is sampled as a logic “one”, the output signal conveyedon node 330 is caused to transition states (i.e., change states from aone to a zero, or from a zero to a one). However, when the phasedetector output is sampled as a logic “zero”, the output signal conveyedon node 330 is left unchanged. Consequently, the signal conveyed on node330 encodes the phase error by the number of transitions in the signalcompared to the number of non-transitions in the signal (i.e., the“transition-density” of the signal).

This signal conveyed on node 330 is next communicated to thedivide-by-2^(N) block 322, which may conveniently be a simple ripplecounter or other binary counter, and which generates on its output node332 a divided-down version of its input signal. Such a “transitiondecimation” circuit generates a transition on its output for every groupof transitions on its input signal. The transition-to-1 converter 324then generates a logic one on its output node 334 upon detecting atransition of its input signal received from the divide-by-2^(N) block322. As can be appreciated, the output signal 334 represents a decimatedversion of the delta-sigma phase error signal: it is likewise an outputsequence of ones and zeros, and the number of ones compared to thenumber of zeros (i.e., the ones-density of the signal) encodes theanalog phase error voltage (and thus may be also viewed as a delta-sigmaencoded signal). However, the effective clock rate of this decimatedsignal is now a factor of 2^(N) slower. Further details of such anarrangement are described in greater detail herebelow.

The digital accumulator 212 is implemented in this particular embodimentas an up/down counter 326. The decimated phase error signal is receivedfrom node 334 and coupled to the UP/DOWN# control input of the up/downcounter 326. An accumulator clock signal conveyed on node 316 ispreferably equal in clock rate to the delta-sigma clock signal dividedby 2^(N). If the ones-density of the input signal received by theup/down counter 326 is greater than 50%, the value of the counter will,over time, increase. Conversely, if the ones-density is less than 50%,the value of the counter will decrease over time. The rate of increase(or decrease) of the digital counter value depends on the degree bywhich the ones density of the input signal exceeds 50% (or is less than50%). Other accumulator structures are contemplated, including a seriesof stages each comprising an adder whose output is latched in aregister. One of the inputs for the adder is taken from the output ofthe register, and the other input is unused. The input signal for theaccumulator is conveyed to the carry in input for the lower-most stage.The respective carry-out signal from each respective stage is coupled tothe respective carry-in input for the succeeding stage, and the registeroutputs for the upper-most stages form the accumulated digital word.

The decimation stage 312 shown in FIG. 12 may be implemented in avariety of ways. One suitable arrangement (implementing, for example, adivide-by-eight decimation stage) is shown in FIG. 13A. Here, the1-to-transition converter 320 is implemented by an XOR gate 340 and aD-register 342 configured in a well-known arrangement, and clocked bythe delta-sigma clock conveyed on node 254. Each stage of thedivide-by-2^(N) block 322 is implemented using a divide-by-two register(e.g., 351, 353, and 355) paired with a D-register (e.g., 352, 354, and356). Such a divide-by-two register may be easily implemented byconnecting a complement Q output signal to its own D input node, orsimilar structures. Each D-register is synchronized to a correspondingdivided-down clock from the delta-sigma clock by a trio of registers344, 345, and 346, each configured to divide by two. The transition-to-1converter 324 is preferably implemented using a D-register 360 and anXOR gate 362 configured in a well-known arrangement, and clocked by thedivided-down delta-sigma clock conveyed on node 316. This samedivided-down clock signal preferably serves as the accumulator clockthat, for this embodiment, is coupled to the up/down counter 326.

As can be appreciated from an inspection of FIG. 13A, very littlecircuitry operates at the relatively fast clock rate of the delta-sigmaclock, and the circuitry that does operate at that rate is very simpleand requires very few propagation delays between clock transitions. Theripple counter quickly lowers the clock rate of each succeeding stage sothat the accumulator clock runs at the delta-sigma clock rate divided by2^(N). For an exemplary embodiment using a 1.25 GHz delta-sigma clockand a divide-by-eight decimation stage, the accumulator clock preferablyruns at only 155 MHz. At this modest speed, an up/down counter or othersuitable digital accumulator structure may be designed using area andpower efficient single-ended logic circuits (rather than fullydifferential circuits) and may be synthesized using commerciallyavailable logic synthesis tools. Consequently, as the value of N in thedecimation stage increases, the power dissipation of the digitalaccumulator is reduced at the expense of lower resolution in theaccumulator path. In an alternative structure, the digital integrationblock shown in FIG. 10 may include an up/down counter connected directlyto the output signal from the delta-sigma modulator conveyed on node222, which counter is clocked at the full delta-sigma clock rate. Such astructure is conceptually simpler but consumes additional power inoperation. While integer divide ratios are likely preferred (e.g.,divide-by-two per stage), other divide ratios of positive rationalnumbers (i.e., a ratio of integers) may also be provided.

Referring now to FIG. 13B, another embodiment suitable for use as adigital accumulator is shown (such as the digital accumulator 212 shownin FIG. 11, or such as the digital integrator 210 shown in FIG. 10).Digital accumulator 580 includes a least-significant-bit (LSB)accumulator 582, here shown as a 7-bit accumulator, and amost-significant-bit (MSB) accumulator 586, here shown as a 12-bitaccumulator. An overflow/underflow block 584 is optionally included togenerate an underflow and overflow signal (i.e., an increment anddecrement signal) for the MSB accumulator 586. The increment/decrementsignals may be generated to reflect an overflow/underflow condition fromany of several bit positions within the LSB accumulator 582, thusproviding for a variable width of the overall digital accumulator 580 asa function of a DATA_RATE_SELECT signal received by theoverflow/underflow block 584. Lower data rates preferably selectincreasingly higher accumulator widths to satisfy jitter requirements ofthe PLL. For example, for a clock and data recovery embodiment describedherein, the digital accumulator 580 width is preferably configured as 19bits for OC-3 data rates (i.e., selecting bit 6 from the LSB accumulatorto generate an overflow or underflow), while for OC-48 the width ispreferably configured as 16 bits (i.e., selecting bit 3 from the LSBaccumulator to generate an overflow or underflow). In otherapplications, the width of the digital accumulator may be fixed and theselectable overflow/underflow block 528 not used, resulting in a fixeddecimation by the LSB accumulator 582 (and possibly decimatedadditionally by a separate preceding decimation stage). In oneembodiment, the signals from each bit position of the LSB accumulator582 are themselves an increment and a decrement signal, and theoverflow/underflow block 584 may be a multiplexer circuit (and theINC/DEC input from the LSB accumulator not utilized. In anotherembodiment, the signal from each bit position of the LSB accumulator 582may be a carry-out signal for the bit position, and theoverflow/underflow block 584 utilizes the INC/DEC input from the LSBaccumulator (as shown) to generate the increment/decrement signal forthe MSB accumulator 586.

Preferably both accumulators 582 (if used) and 586 may be loaded with avalue and/or “frozen” during calibration and test modes. For example,the accumulators are preferably loaded with a value at or near itsmid-point value and frozen during calibration, and then released whenattempting to acquire lock. To enhance testing capabilities, theaccumulators are preferably loadable with an arbitrary value in a testmode.

Irrespective of the configured width of the digital accumulator 580, theoutput conveyed to the DAC, for this exemplary embodiment, remains as a12-bit output from the MSB accumulator 586, which may be clocked usingthe same clock as for the LSB accumulator 582 (as shown), or may beclocked at a slower clock rate to save power. For example, the MSBaccumulator 586 may be clocked at a constant rate that is lower than theclock rate for the LSB accumulator 582, or may vary in accordance withwhich output bit is selected by the multiplexer 584.

The accumulator 580 may be connected to the output of a separatedecimation circuit, such as the decimation circuit 312 shown in FIG. 12,or may be connected directly to the output of a digital phase detectorwith the LSB accumulator 582 essentially functioning as a decimationcircuit in its own right. In a broader sense, the digital accumulatormay be bifurcated into a LSB-portion operating a clock rate, and a MSBportion operating at a lower clock rate than the LSB portion. The databits of the LSB portion may be ignored by downstream circuitry, thusperforming a decimation function by preserving the width of the digitalaccumulator but decreasing the output resolution of the accumulator. Inan even broader sense, the digital accumulator may be segmented intomore than two hierarchical sections, with higher order sectionspreferably (but not necessarily) operating at a lower clock rate thanpreceding sections. For example, each respective higher order sectionsmay operate at a lower clock rate than the respective preceding section,although such is not necessarily required. In one embodiment, a cascadedseries of 1-bit adders may implement a decimation circuit or a portion(or all) of a digital accumulator circuit. Some number of the lowerorder data bits (irrespective of which hierarchical section they reside)may be ignored by downstream circuitry, thus performing a decimationfunction by preserving the width of the digital accumulator butdecreasing the output resolution of the accumulator. Alternatively, allthe digital accumulator bits may be presented to and acted upon bydownstream circuitry, thus performing no decimation function within theaccumulator. Such a structure may be implemented with or without anadditional decimation circuit coupled between the phase detector outputand the digital accumulator input. In certain embodiments, a singledigital accumulator may be configurable to different lengths, but whoseoutput word is a variable length beginning with the least significantbit, to provide a variable dynamic range but with the same effective“capacitance” value. In any of these structures, any decimated bits formpart of the digitally-accumulated word, and thus in a broad sense, adigital accumulator circuit may be thought of as potentially including,but certainly not requiring, a separate decimation circuit.

Referring again to FIG. 5, the D/A converter 214 may be implementedusing any convenient structure, but preferably is implemented using adelta-sigma converter sampled at the accumulator clock rate. Adelta-sigma D/A converter is advantageous here because of its highanalog output voltage (i.e., conversion) accuracy, its small physicalsize, its ease of design and implementation, its low power, and thetolerance (within an integrating path synthesizing a low-pass filter)for a relatively slow conversion time. Such circuits are well known inthe art, and a detailed description is unnecessary. The filter block 216connected to the output of the D/A converter 214 may be any suitablefilter structure, and may conveniently be an RC filter implemented usinga modest-sized capacitor only a few tens of picofarads in size whichneed only be a low-Q capacitor (e.g., may be implemented using an MOStransistor gate capacitance).

Referring now to FIG. 13C, a block diagram is shown of an exemplaryportion of a PLL circuit including a digital phase detector and adigitally-synthesized loop filter capacitor, similar to thatconceptually depicted in FIG. 5. The phase locked loop portion 600includes a digital phase detector 628 which receives the input datasignal conveyed on node 220 and a data rate clock signal conveyed onnode 236. The digital phase detector 628 generates on its output node606 an error signal which digitally encodes the phase difference betweenthe input data signal and the data rate clock signal. In this embodimentthe digital phase detector includes a linear phase detector 602 and adelta-sigma modulator 604. The digital phase detector 628 may alsoinclude additional circuitry to generate on an output node (not shown)the reconstructed output data.

The loop filter for this exemplary PLL portion 600 includes a feedforward path formed by a charge pump 608 responsive to the delta-sigmasignal on node 606, followed by a configurable gain stage 610 and alow-pass filter block 612 having, for this embodiment, a preferredbandwidth of approximately 40 MHz. The configurable gain stage 610 ispreferably a configurable current mirror which provides a gain which isselectable between about 0.5 to about 1.5. With a 5-bit digital wordused to convey the desired gain, a configurable gain stage 610 ispreferably configured so that the gain is equal to(24+g₀2⁰+g₁2¹+g₂2²+g₃2³+g₄2⁴)/ 40.

The loop filter for this exemplary PLL portion 600 further includes adigital integrating block 632. The output signal of the feed forwardpath is combined with the output signal of the digital integrating blockon a combined output node 623, which is converted by amplifier 624 andfeedback resistor 625 to a voltage signal suitable for use as a controlsignal for a voltage controlled oscillator. A voltage reference conveyedon node 636 is preferably about one volt in magnitude and is preferablygenerated by an internal band-gap reference circuit.

The digital integrating block 632 includes a decimator block 614, adigital accumulator 616, a sigma-delta D/A converter 618, a charge pump620, and a low-pass filter 622. The decimator 614 receives the digitalphase error representation from the digital phase detector 628 andgenerates a decimated phase error signal, which is then integrated bythe digital accumulator 616. The digital accumulator 616 preferably maybe configured as in the accumulator 580 shown in FIG. 13B. Themultiple-bit register within the accumulator 616 representing theintegrated value of the phase error is preferably communicated on anN-bit wide output bus to the D/A converter 618 which, along with thecharge pump 620, converts the digital representation into an analogsignal which is then filtered by the low-pass filter 622 and combinedwith the feedforward path signal at node 623. The combined output isthen converted to a voltage to form an analog control voltage on outputnode 626. The decimator 614 may be configured to generate theaccumulator clock (e.g., a 155 MHz clock signal) as well as other lowfrequency clocks (conveyed on node(s) 634) which are useful andconvenient for other portions of an integrated circuit. The MID_RAILsignal is preferably asserted to effectively “disconnect” thefeedforward path during calibration.

Referring now to FIG. 14, a block diagram of an exemplary clock and datarecovery circuit 400 is shown which advantageously incorporates thepresent invention. This exemplary clock and data recovery circuit 400 ispreferably implemented as a single integrated circuit particularly wellsuited to operation with a SONET data stream. A differential input datasignal is buffered and conveyed to a phase detector 402 along with adata rate clock signal conveyed on node 412. The phase error signal fromthe phase detector 402 is filtered by a loop filter 404, such asdescribed above, to generate a first control signal for a VCO 406. Aclock divider block 408 selectively divides the VCO clock signal inaccordance with an externally-provided rate select signal communicatedon node 416, to generate the data rate clock signal on node 412. Anexternally-provided reference clock signal REFCLK is buffered andconveyed on node 414 to a lock detector block 410, along with the rateselect signal on node 416 and the VCO clock signal. The lock detectorblock 410 generates a LOSS-OF-LOCK signal (LOL) and also generates asecond control signal for the VCO 406 (on node 418) to aid in frequencyacquisition of the data recovery PLL. When the lock to data is lost, theclock and data recovery PLL is preferably tuned to the reference clockREFCLK instead. The VCO 406 is preferably an LC oscillator, althoughother types are also contemplated. Exemplary LC oscillators aredescribed in U.S. Pat. No. 6,137,372 to Welland (which describes certaincalibration capabilities of such an oscillator) which is herebyincorporated by reference, and further described in “Feedback SystemIncorporating Slow Digital Switching for Glitch-Free State Changes,” byRex T. Baird, et. al., U.S. Provisional Application No. 60/300,699,filed on Jun. 25, 2001, which application is hereby incorporated byreference. Other types of controlled oscillators, such as a currentcontrolled oscillator with a suitable control signal, are alsocontemplated.

Other preferred circuits useful for implementing a phase locked loopcircuit, and particularly suited for a clock and data recoveryapplication, are described in the following U.S. patent applications,each of which is being filed on Jul. 10, 2001, and each of which ishereby incorporated by reference: “Digital Phase Detector Circuit andMethod Therefor,” by Perrott, U.S. patent application Ser. No.09/902,542; and “Apparatus and Method for Decimating a Digital InputSignal,” by Perrott, U.S. patent application Ser. No. 09/902,548.

An auto-detect block is preferably implemented within the lock detectorblock 410 to auto-detect which REFCLK frequency, of several possiblefrequencies, is received by the device, without requiring dedicatedintegrated circuit pins to so indicate. Alternatively, of course,dedicated external pins may be used to communicate the particular REFCLKfrequency being used. In this exemplary circuit 400, the VCO 406 isconfigured to operate at a nominal frequency of 2.488 GHz when receivingan externally-provided REFCLK frequency of either 155.52, 77.76, or19.44 MHz. In operation, the exact frequency of the VCO 406 adjusts tothat of the incoming data signal. Depending upon which REFCLK frequencyis detected, various dividers and logic gates are configured in the lockdetector 410 to ensure that the VCO operates at a multiple of the REFCLKfrequency necessary to generate a 2.488 GHz clock rate, as noted inTable 1 below. An exemplary auto-detection capability is described in“Integrated Circuit Incorporating Auto-Detection of anExternally-Provided Reference Clock Frequency and Method Therefor” byMichael H. Perrott, et al, U.S. patent application 09/902,543, filed onJul. 10, 2001, and which is hereby incorporated by reference.

TABLE 1 SONET/ Bit Gigabit Ratio of VCO SDH Rate Ethernet to REFCLK19.44 MHz 19.53 MHz 20.83 MHz 128 77.76 MHz 78.125 MHz 83.31 MHz  32155.52 MHz 156.25 MHz 166.63 MHz  16

The phase detector 402 is preferably implemented using a digital phase202 shown in FIG. 6 incorporating a delta-sigma modulator. The dataclock on node 412 to the phase detector 402 may be up to 2.5 GHz infrequency. The delta-sigma clock provided to the phase detector 402 ispreferably generated at 1.25 GHz. The loop filter 404 preferablyincludes a digital integrating block as described above, whichadvantageously includes a 2³ decimator, such as decimator block 312, andadvantageously includes a digital accumulator, such as digitalaccumulator 212, having at least 12 bits of resolution and clocked at arate equal to the delta-sigma clock rate divided by the decimationfactor (e.g., up to 155 MHz).

In an exemplary embodiment, a clock and data recovery integrated circuitis configured to operate at any of four general data rates, as recitedin Table 2.

TABLE 2 OC48 with RATESEL SONET/ Bit Gigabit 15/14 Data CLK [0:1] SDHRate Ethernet FEC Divider 00 OC48 2.488 Gbps — 2.67  1 Gbps 10 — 1.244Gbps 1.244 Gbps —  2 01 OC12 622.08 Mbps — —  4 11 OC3 155.52 Mbps — —16

As used herein, a “clock signal” is not necessarily a well-shaped squarewave with abrupt transitions, as is commonly assumed in modest-speeddigital circuits. Rather, a clock signal need only be a periodic signal(or a gated periodic signal). Consequently, sawtooth waveforms, “sloppy”square waveforms, sinusoidal waveforms, triangular waveforms, and anyother periodic waveform may be used as a clock signal. Anexternally-provided frequency reference signal may be a signal entirelygenerated off-chip and conveyed as an identifiable signal to theintegrated circuit. Alternatively, such an externally-provided frequencyreference signal may be provided by a resonant circuit coupled to theintegrated circuit, such as a crystal, even though a portion of anyrequired “oscillator” circuitry may be contained on-chip. Moreover, asused herein, the term “decimation” does not necessarily refer to a powerof ten as its Latin name might imply, but is used, as is common in theart, to refer to any amount of discarding of an input signal in favor ofkeeping a remaining portion. A data interval is the period of a singledata bit. A latching circuit may be a register or a latch as understoodby one skilled in the art, and may be level sensitive or edge-triggeredon its clock input. Many suitable latching circuits, both single-endedand differential, are well known in the art. As used herein, a digitalaccumulator circuit is intended to be afforded the broadest possibleinterpretation, to encompass a wide variety of circuits which may beemployed to accomplish the desired function.

The invention is not contemplated to be limited to traditional siliconsemiconductor technologies, as other suitable semiconductortechnologies, such as gallium arsenide, silicon carbide, and indiumphosphide may take advantage of the teachings herein.

While the invention has been largely described with respect to theembodiments set forth above, the invention is not necessarily limited tothese embodiments. Variations and modifications of the embodimentsdisclosed herein may be made based on the description set forth herein,without departing from the scope and spirit of the invention as setforth in the following claims. For example, although a preferred analog(i.e., linear) phase detector is described, the invention is not limitedto such a phase detector. Other configurations may be used and stillenjoy the advantages described of using a digital integration block. Forexample, for certain applications a bang-bang phase detector (describedabove) may be employed. Moreover, the invention is not limited to usewith a first-order delta-sigma modulator. Higher order delta-sigmamodulators may be used. Moreover, other digital encoding schemes,requiring a modulator other than a delta-sigma modulator, may also beadvantageously incorporated without departing from the spirit of theinvention. Nor is the digital encoder used limited to a one-bit outputmodulator. For example, a multi-bit output delta-sigma modulator orother digital modulators or encoders may be used advantageously. Thepresent invention is useful in many types of feedback systems andprovides an improved way to achieve a long time constant withoutrequiring a large, traditional capacitor. Accordingly, otherembodiments, variations, and improvements not described herein are notnecessarily excluded from the scope of the invention, which is definedby the following appended claims.

A particular advantage of using a digitally-synthesized loop filter asdescribed herein within a phase locked loop circuit is reduced drift ofthe control signal for the oscillator within the loop, and consequentlybetter frequency stability. Since the control signal is derived from a“filtered” value maintained in a digital accumulator rather than as ananalog voltage on a node, a long string of transition-less bits causesmuch less drift on the control signal ultimately presented thecontrolled oscillator of the phase locked loop.

Even though the preferred embodiments are described in the context of aphase locked loop circuit arranged for clock and data recovery, itshould be appreciated that such a circuit is not necessarily requiredunless specifically enumerated in a particular claim. The teachings ofthe present invention are believed advantageous for use with other typesof circuits, such as a reference-less phase locked loop circuit. A loopfilter feedforward path as described herein may also be implementeddigitally using the teachings set forth herein. Moreover, in certainembodiments, the digital accumulator value may be directly used tocontrol a VCO or other controlled circuit without being first convertedto an analog signal. For example, a digitally-accumulated word (orportion thereof) may be used to control a plurality of slow-switchedcapacitor circuits for an LC oscillator, as described in “FeedbackSystem Incorporating Slow Digital Switching for Glitch-Free StateChanges,” by Rex T. Baird, et. al., U.S. Provisional Application No.60/300,699, filed on Jun. 25, 2001, and incorporated herein byreference. Moreover, for certain applications, such capacitor circuitscould also be switched abruptly, particularly if system jitterspecifications allow such changes. In an abstract sense, such VCOcapacitor control circuits may be thought of as providing a D/A andfilter operation for the digital accumulator circuit, particularly ifthe VCO incorporates slow switching.

Alternatively, an analog “current signal” may be provided rather than avoltage signal to control an oscillator or other controlled circuit. Thecontrol signal may take any of a variety of forms, depending on thecircuit being controlled. Accordingly, other embodiments, variations,and improvements not described herein are not necessarily excluded fromthe scope of the invention, which is defined by the following appendedclaims.

Based upon the teachings of this disclosure, it is expected that one ofordinary skill in the art will be readily able to practice the presentinvention. The descriptions of the various embodiments provided hereinare believed to provide ample insight and details of the presentinvention to enable one of ordinary skill to practice the invention.Although certain supporting circuits (e.g., VCOs, RC filters, adderblocks, gain blocks, input/output buffers, etc.) are not specificallydescribed, such circuits are well known, and no particular advantage isbelieved to be afforded by specific variations of such circuits in thecontext of practicing this invention. Moreover, it is believed that oneof ordinary skill in the art, equipped with the teaching of thisdisclosure, will be able to carry out the invention, includingimplementing various other circuits not specifically described herein,using well known circuit techniques and without undue experimentation.

General Terminology

Regarding general terminology used herein, it will be appreciated by oneskilled in the art that any of several expressions may be equally wellused when describing the operation of a circuit including the varioussignals and nodes within the circuit. Any kind of signal, whether alogic signal or a more general analog signal, takes the physical form ofa voltage level (or for some circuit technologies, a current level) of anode within the circuit. It may be correct to think of signals beingconveyed on wires or buses. For example, one might describe a particularcircuit operation as “the output of circuit 10 drives the voltage ofnode 11 toward VDD, thus asserting the signal OUT conveyed on node 11.”This is an accurate, albeit somewhat cumbersome expression.Consequently, it is well known in the art to equally describe such acircuit operation as “circuit 10 drives node 11 high,” as well as “node11 is brought high by circuit 10,” “circuit 10 pulls the OUT signalhigh” and “circuit 10 drives OUT high.” Such shorthand phrases fordescribing circuit operation are more efficient to communicate detailsof circuit operation, particularly because the schematic diagrams in thefigures clearly associate various signal names with the correspondingcircuit blocks and node names. For convenience, an otherwise unnamednode conveying the CLK signal may be referred to as the CLK node.Similarly, phrases such as “pull high,” “drive high,” and “charge” aregenerally synonymous unless otherwise distinguished, as are the phrases“pull low,” “drive low,” and “discharge.” It is believed that use ofthese more concise descriptive expressions enhances clarity and teachingof the disclosure. It is to be appreciated by those skilled in the artthat each of these and other similar phrases may be interchangeably usedto describe common circuit operation, and no subtle inferences should beread into varied usage within this description.

As an additional example, a logic signal has an active level and aninactive level (at least for traditional binary logic signals) and theactive and inactive levels are sometimes also respectively called activeand inactive “states.” The active level for some logic signals is a highlevel (i.e., an “active-high” signal) and for others is a low level(i.e., an “active-low” signal). A logic signal is “asserted” or“activated” when driven to the active level. Conversely, a logic signalis “de-asserted” or “de-activated” when driven to the inactive level. Ahigh logic level is frequently referred to as a logic “1” and a lowlogic level is frequently referred to as a logic “0” (at least forpositive logic).

Frequently logic signals are named in a fashion to convey which level isthe active level. For example, CLKEN is commonly used to name anactive-high clock enable signal, because the true polarity is implied inthe name. Conversely, CLKENB, /CLKEN, CLKEN#, CLKEN*, CLKEN_L, CLKEN_C,or #CLKEN are commonly used to name an active-low clock enable signal,because one of the many common expressions indicating the complementpolarity is used in the name. It is to be appreciated by those skilledin the art that these and other similar phrases may be used to name thesignals and nodes. The schematic diagrams and accompanying descriptionof the signals and nodes should in context be clear.

Regarding power supplies, a single positive power supply voltage (e.g.,a 2.5 volt power supply) used to power a circuit is frequently named the“VDD” power supply. In an integrated circuit, transistors and othercircuit elements are actually connected to a VDD terminal or a VDD node,which is then operably connected to the VDD power supply. The colloquialuse of phrases such as “tied to VDD” or “connected to VDD” is understoodto mean “connected to the VDD node”, which is typically then operablyconnected to actually receive the VDD power supply voltage during use ofthe integrated circuit. The term may appear either using subscripts(e.g., V_(DD)) or not.

The reference voltage for such a single power supply circuit isfrequently called “VSS.” Transistors and other circuit elements areactually connected to a VSS terminal or a VSS node, which is thenoperably connected to the VSS power supply during use of the integratedcircuit. Frequently the VSS terminal is connected to a ground referencepotential, or just “ground.” Describing a node which is “grounded” by aparticular transistor or circuit (unless otherwise defined) means thesame as being “pulled low” or “pulled to ground” by the transistor orcircuit. Describing a circuit as functioning with a “VDD supply” and“ground” does not necessarily mean the circuit cannot function usingother power supply potentials.

The block diagrams herein may be described using the terminology of asingle node connecting the blocks. Nonetheless, it should be appreciatedthat, when required by the context, such a “node” may actually representa pair of nodes for conveying a differential signal, or may representmultiple separate wires (e.g., a bus) for carrying several relatedsignals or for carrying a plurality of signals forming a digital word.

While the invention has been largely described with respect to theembodiments set forth above, the invention is not necessarily limited tothese embodiments. Variations and modifications of the embodimentsdisclosed herein may be made based on the description set forth herein,without departing from the scope and spirit of the invention as setforth in the following claims. Accordingly, other embodiments,variations, and improvements not described herein are not necessarilyexcluded from the scope of the invention, which is defined by the

What is claimed is:
 1. In a phase locked loop system, adigitally-synthesized loop filter circuit comprising: a first circuitfor providing a digital representation of a phase error signal of thephase locked loop; a second circuit responsive to the digital phaseerror signal, for generating a multi-bit accumulated digital phase errorsignal representing an accumulated value of successive values of thedigital phase error signal; and a third circuit for generating an outputsignal corresponding to the accumulated digital phase error signal, saidoutput signal useful for controlling an oscillator within the phaselocked loop.
 2. The invention defined by claim 1 wherein the firstcircuit comprises: a digital phase detector circuit for the phase lockedloop system.
 3. The invention defined by claim 2 wherein the digitalphase error signal comprises: a quantized-time and quantized-valuesignal which encodes polarity of phase error.
 4. The invention definedby claim 2 wherein the digital phase error signal comprises: aquantized-time and quantized-value signal which linearly encodes phaseerror.
 5. The invention defined by claim 1 wherein the first circuitcomprises: an analog-to-digital converter circuit for converting ananalog representation of a phase error signal into a correspondingdigital representation.
 6. The invention defined by claim 5 wherein thefirst circuit comprises: a delta-sigma encoder circuit having a clockinput coupled to receive a first clock signal.
 7. The invention definedby claim 1 wherein the second circuit comprises: a digital accumulatorcircuit for accumulating successive values of a digital input signalrepresenting the phase error, and for providing on an output thereof amulti-bit accumulated digital phase error signal.
 8. The inventiondefined by claim 7 wherein the digital accumulator circuit comprises: anaccumulator circuit for conveying the multi-bit accumulated digitalphase error signal having a variable number of bits beginning with theleast significant bit.
 9. The invention defined by claim 7 wherein thedigital accumulator circuit comprises: a lower-order accumulator circuithaving an input responsive to the digital input signal representing thephase error, and having at least one output; and an upper-orderaccumulator circuit having an input coupled to an output of thelower-order accumulator circuit, and having an output for conveying aportion of the multi-bit accumulated digital phase error signal; whereinboth the lower-order accumulator circuit and the upper-order accumulatorcircuit together compute the multi-bit accumulated digital phase errorsignal.
 10. The invention defined by claim 9 wherein: the upper-orderaccumulator circuit is clocked at a lower rate than the lower-orderaccumulator circuit is clocked.
 11. The invention defined by claim 9wherein: of the upper-order and lower-order accumulator bits forming theaccumulated digital phase error signal, only a portion of the bits areconveyed to the third circuit.
 12. The invention defined by claim 11wherein: of the upper-order and lower-order accumulator bits forming theaccumulated digital phase error signal, only upper-order accumulatorbits are conveyed to the third circuit.
 13. The invention defined byclaim 7 wherein: the digital phase error signal from the first circuitis coupled to the digital accumulator circuit as the digital inputsignal.
 14. The invention defined by claim 13 wherein the second circuitcomprises: a digital accumulator circuit having an input coupled toreceive the digital phase error signal, having a clock input coupled toreceive an accumulator clock signal, and having an output for conveyinga multi-bit output signal representing an accumulated value ofsuccessive values of the digital phase error signal.
 15. The inventiondefined by claim 7 wherein: the digital input signal for the digitalaccumulator circuit is derived from the digital phase error signalreceived from the first circuit.
 16. The invention defined by claim 15wherein the second circuit further comprises: a decimator circuitresponsive to the digital phase error signal, for generating a decimateddigital phase error signal which is coupled to the accumulator circuitas the digital input signal.
 17. The invention defined by claim 1wherein: the output signal comprises an analog representation of atleast a most-significant-bit portion of the accumulated digital phaseerror signal.
 18. The invention defined by claim 1 wherein: themulti-bit accumulated digital phase error signal is computed to agreater number of bits than is conveyed to the third circuit.
 19. Theinvention defined by claim 1 wherein the third circuit comprises: adigital-to-analog converter circuit.
 20. The invention defined by claim19 wherein the third circuit further comprises: a low-pass filtercircuit connected to an output of the digital-to-analog convertercircuit.
 21. The invention defined by claim 1 wherein: an effectivevalue of capacitance provided by the digitally-synthesized loop filtercircuit increases with increasing numbers of bits in the accumulateddigital phase error signal.
 22. The invention defined by claim 21wherein: the number of bits in the accumulated digital phase errorsignal is configurable.
 23. The invention defined by claim 22 wherein:the number of bits in the accumulated digital phase error signal isconfigurable to alter an operating characteristic of the phase lockedloop system.
 24. The invention defined by claim 1 wherein: the loopfilter circuit is implemented entirely within an integrated circuit. 25.The invention defined by claim 1 wherein: the loop filter is implementedentirely within an integrated circuit configured to recover clock anddata from an incoming data signal having more than one possible datarate; and the number of bits in the accumulated digital phase errorsignal is configurable based upon the data rate of the incoming datainput signal.
 26. The invention defined by claim 1 wherein: the firstcircuit comprises a digital encoder circuit forming a portion of a phasedetector circuit for the phase locked loop system, said digital encodercircuit having a clock input coupled to receive a first clock signal.27. The invention defined by claim 26 wherein the second circuitcomprises: a decimator circuit responsive to the digital phase errorsignal from the first circuit, for generating a decimated digital phaseerror signal having a lower data rate than the digital phase errorsignal; and a digital accumulator circuit for accumulating successivevalues of the decimated phase error signal, having a clock input coupledto receive a second clock signal having a clock rate lower than that ofthe first clock signal, and for providing on an output thereof at leasta most-significant-bit portion of the multi-bit accumulated digitalphase error signal.
 28. The invention defined by claim 27 wherein thethird circuit comprises: a digital-to-analog converter circuit having aninput responsive to at least the most-significant-bit portion of themulti-bit accumulated digital phase error signal, and having an output;and a filter having an input connected to the output of thedigital-to-analog converter circuit, and having an output for providingthe output signal corresponding to the accumulated digital phase errorsignal.